Process for purifying gases containing sulphur compounds and recovering elemental sulphur

ABSTRACT

1. IN A PROCESS FOR PURIFYING A GAS CONTAINING H2S, SO2 AND ORGANIC SULPHUR COMPOUNDS, SAID PROCESS COMPRISING REACTING H2S AND SO2 IN A SULPHUR PRODUCING REACTION WITH AN ORGANIC SOLVENT AT 20-160*C., THE IMPROVEMENT COMPRISING THE PRELIMINARY STEPS OF (A) REACTING SAID GAS AND STEAM IN A FIRST REACTION ZONE IN THE PRESENCE OF A CATALYST CONSISTING ESSENTIALLY OF ALUMINA AT 250-400*C. TO CONVERT SAID ORGANIC COMPOUNDS SUBSTANTIALLY COMPLETELY TO H2S WHILE EFFECTING SUBSTANTIALLY NO CONVERSION OF THE SO2 IN THE GAS TO H2S, RESULTING IN A GAS HAVING AN UNDESIRABLY HIGH MOLAR RATIO OF H2S/SO2, AND (B) REACTING RESULTANT GAS FROM STEP (A) WITH SUFFICIENT OXYGEN IN A SECOND REACTION ZONE AT 300-500*C., TO CONVERT A PORTION OF THE H2S IN SAID GAS TO SO2 AND TO FORM A GAS HAVING A DESIRABLY LOWER MOLAR RATIO OF H2S/SO2 BETWEEN 1.6 AND 3.

United States Patent 3,845,197 PROCESS FOR PURIFYTNG GASES CONTAINING SULPHUR COMPOUNDS AND RECOVERING ELEMENTAL SULPHUR Philippe Renault, Noisy-le-Roi, Andre Deschamps, Chatou, and Claude Dezael, Maisons-Laflitte, France, assignors to Institut Francais du Petrole, des Carburants et Lubrifiants No Drawing. Filed Jan. 24, 1972, Ser. No. 220,403 Claims priority, application France, Jan. 29, 1971, 7103181 Int. C1. C01!) 17/04 US. Cl. 423575 20 Claims ABSTRACT OF THE DISCLOSURE Process for purifying a gas stream containing sulphur compounds such as COS and CS in which said stream is passed, together with steam, at 250-400 C., over an alumina containing catalyst so as to produce H S, a portion of the produced H is oxidized to S0 at 300- 500 C., so as to obtain a ratio H S/S0 from 1.6 to 3, and said S0 is reacted with the remaining portion of H 5 at 20160 C., so as to produce elemental sulphur.

This invention relates to an improved process for purifying gas streams containing sulphur compounds such as COS and CS and recovering elemental sulphur therefrom.

The gas streams flowing out from plants of the Claus type contain, even after several catalytic treatments, significant amounts of H 5 and S0 and sometimes organic sulphur compounds such as COS and CS Generally they contain from 0.01% to 4% (egg. from 0.1 to 4%) by volume of sulphur compounds, from 5 to 60% (e.g. from to 40%) of steam and in some cases a small amount of hydrogen, e.g. 0.1 to 3%. In the literature, e.g., U.S. Pats. 3,441,379 and 3,598,529, there is described the conversion of hydrogen sulphide and sulphurous anhydride to sulphur in the presence of a solvent according to the following reaction:

Under these conditions, the organic sulphur compounds do not undergo a sufficient conversion to provide an efiiuent entirely free from sulphur. It is an object of this invention to improve the final purification by converting the organic sulphur compounds to hydrogen sulphide and by restoring a ratio of the hydrogen sulphide to the sulphurous anhydride (S0 of from 1.6 to 3 and preferably from 1.8 to 2.4.

The contemplated process consists of purifying the gas efiiuents containing from 0.01 to 4% (preferably from 0.1 to 4%) by volume of sulphur compounds such as COS and CS and from 5 to 60% (preferably from 10 to 40%) of steam, with optionally from 0.1 to 3% of hydrogen and in some cases H 8 and /or S0 According to this process, in a first reaction zone, a gas containing COS and/or CS and steam is caused to pass, at a temperature from 250 C. to 400 C., over an alumina containing catalyst.

In a second reaction zone, a portion of the hydrogen sulphide produced in the preceding step is oxidized to sulphurous anhydride so as to obtain a ratio of the hydrogen sulphide to the sulphurous anhydride of from 1.6 to 3, preferably from 1.8 to 2.4 and particularly from 1.9 to 2.2. This operation is conducted at a temperature of 300 to 500 C. while adding oxygen to the outflow from the first reaction zone, said oxygen being used as such or as an oxygen-containing gas in an amount of, for example, from 0.1 to 5% of the total reaction volume; then, in a third reaction zone, hydrogen sulphide is reacted with sulphurous anhydride at a temperature from 20 to 160 C. (preferably from to C.) in the presence of an organic solvent and elemental sulphur is recovered at the outlet of the third zone.

This process may be used more particularly for the treatment of the gas efiiucnts from Claus units generally containing (by volume): from 0.1 to 3% of hydrogen sulphide and sulphurous anhydride in a ratio of hydrogen sulphide to sulphurous anhydride close to 2, from 0.01 to 0.3% of COS, from 0.01 to 3% of CS elemental sulphur and other components consisting mainly of nitrogen, steam and carbon dioxide.

The outflow from a unit of the Claus type may also be treated by an inert organic solvent, as described in US. Pat. 3,598,529. The product of this treatment contains only very small amounts of H 8 and S0 but it still contains such compounds as COS and CS which are converted only to a minor extent by said treatment. The treatment of this invention is to be applied to such an outflow.

In the first reaction zone, according to the present invention, there is achieved the conversion to hydrogen sulphide of the sulphur organic compounds (CS COS, contained in the gas efiiuent, at a temperature of, for example, from 250 to 400 C., preferably from 300 to 350 C., over an alumina containing catalyst having preferably an alkali metal content lower than 0.1%, a specific surface from 40 to 500 m. /g., more preferably from to 350 m ./g., and a pore volume from 10 to 80 cc./100 g. and more preferably from 30 to 60 cc./l00 g. This alumina may be used alone but it is advantageous in some cases, to deposit on the alumina one or more metals selected from the metals of Groups VI-A and VIII of the periodic classification of elements as published by Societ Chimique de France, and more particularly selected from the group consisting of molybdenum, tungsten and the metals pertaining to the iron group (iron, nickel and cobalt). In this latter case the content of metals from Groups VI-A and VIII is preferably from 1 to 20% by weight, expressed as oxide.

The catalyst containing metals from Groups VI-A and VIII is preferably used in the sulphurized form. As examples of associations of metals from Groups VI-A and VIII, there can be mentioned: cobalt-molybdenum, cobalttungsten, nickel-molybdenum and nickel-tungsten.

Although the pressure may be selected within a wide range, the operating pressure, in the first reaction zone, may be close to the atmospheric pressure, for example from 1.05 to 2 absolute atmospheres. The space velocity (V.V.H.=volume of gas introduced per hour divided by the volume of the catalyst) depends on the content of sulphur compounds in the treated etfluent; it is generally from 500 to 5000.

The gas outflow from the first stage no longer contains substantial amounts of sulphur organic compounds.

This outflow is then conveyed to a second reaction zone in which there is achieved a controlled oxidation by oxygen of a portion of the hydrogen sulphide in the gas stream. The oxidation is conducted according to the reaction mechanism:

112s +-g- 02 so. 11.0

For this purpose, an oxygen-containing gas is admixed to the efliuent. The oxidation of the hydrogen sulphide is conducted at a temperature for example from 300 to 500 C. and preferably from 350 to 400 C. The oxygen flow rate depends on the content in H 8 and other combustible compounds, for example CO or H of the gas. In most of the cases, it amounts to 0.1 to 5% of the gas flow issued from the first reaction zone and it is such that, in the outflow from the second zone, the molar ratio hydrogen sulphide/sulfurous anhydride be from 1.6 to 3, preferably from 1.8 to 2.4, for example from 1.9 to 2.2. It has been discovered, and this is one of the objects of the invention, that the best purification 5 yields are achieved when the ratio H s/S is within these ranges.

In this second reaction zone, the operating pressure will be preferably from 1.05 to 2 absolute atmospheres, these values being however not limitative.

It may be advantageous to carry out the oxidation in the presence of an oxidation catalyst; this catalyst may, for example, comprise alumina in which one or more metals from Groups V-A, VI-A and VIII of the periodic classification, more particularly selected from chromium, vanadium and iron, are present as oxides, for example in an amount of 1 to 20% by weight.

As examples of metal combination, there can be mentioned: iron-chromium and vanadium-iron.

The oxygen amount at the inlet will be preferably so selected that the outflow from the second reaction zone have a maximum oxygen, content of 700 p.p.m.

The outflow from the second zone, in which the ratio HgS/SO is close to 2, is conveyed to a third reaction zone where the gas is contacted at a temperature of, for example, from 20 to 160 C. and preferably from 115 to 160 C., with an organic solvent stable at the contact temperature. This solvent preferably contains a catalyst favoring the reaction between H 8 and S0 A very wide range of solvents can be used in this reaction zone; there can be mentioned particularly sulfolane, the heavy alcohols having for example from 12 to 20 carbon atoms and more generally the organic liquids which are inert with respect to the treated gas. The presence of water in small amount is favorable. It is however preferred to make use of solvents of the two following types;

the triesters of orthophosphoric acid such as, for example,

tri butyl phosphate, tri n propyl phosphate, triisobutyl phosphate, tri phenyl phosphate and tricyclo hexyl phosphate;

the alkylene-glycols, the poly-alkylene-glycols, the ethers, esters and ether-esters of alkylene-glycols, the ethers, esters, ether-esters of poly-alkylene-glycols, particularly ethylene glycol, tri ethylene glycol, dipropyleneglycol, tetra butylene glycol, deca ethylene glycol mono ethyl ether and poly ethylene glycol having a molecular weight of about 400.

The catalyst, which is advantageously added, is preferably selected among the alkali metal compounds and more particularly among the salts formed between the metals of Groups I and II, left column (Groups I-A and II-A) of the periodic classification and the organic carboxylic acids or partial esters thereof.

The applicant has described such catalysts in the French Pat. No. 1592092 and in US. Pat. No. 3,598,529. By way of example of the preferred compounds which can be used according to the invention, there can be mentioned potassium benzoate, sodium benzoate, potassium salicylate, the sodium and potassium salts of nicotinic acid, cinnamic acid and furoic acid, potassium adipate, acid or neutral potassium phthalates.

When using an organic polyvalent acid for preparing the salt, it is advantageous to partially esterify it before salifying it with an alkali metal or alkaline earth hydroxide, by means of an alcohol compound used as solvent. The applicant has described such catalysts in the French patent application No. PV 7043077 filed on Nov. 30, 1970. Other catalysts which can be used are for example, ammonia, amines, alkanolamines, alkali metal or alkaline earth hydroxides and the alkali metal salts of weak inorganic acids. The reaction medium generally contains grim 0.0; to by weight of catalyst and preferably from The following examples are given for illustrating the invention.

EXAMPLE 1 10,000 Nm /h. (Nm /h. indicates that the volume is measured under normal conditions of temperature and pressure) of an outflow from a Claus unit and whose composition expressed by volume is as follows:

are passed, at a temperature of 300 C, and under a pressure of 1.25 absolute atmospheres, over 10 m. of an alumina containing less than 0.1% of Na O and having a specific surface of 27 7 m./ g.

The analysis of the sulphur compounds of the so-treated outflow is as follows by volume:

H S=l.05% COS=0.01% SO =0.35% CS2=0% This efiluent is partially oxidized at 400 C. under a pressure of 1.15 absolute atmospheres by addition of 750 mfi/h. of air in the presence of an alumina having a specific surface of 277 m. /g. on which 10% by weight of V 0 are deposited.

The analysis of the acid gas from the produced eflluent shows that the acid gas content is by volume as follows:

H S=0.93%; SO =0.455%.

This efiluent is conveyed to a third reaction zone in which it is contacted at C., under a pressure of 1.05 atmospheres, with a polyethylene-glycol having a mean molecular weight of 400 and containing 6.4 g. of potassium benzoate per kg. of solvent. The outflow from said zone then contains no more than 1,400 p.p.m. of acid gas H 8 and S0 which corresponds, after a conventional incineration, to about 1000 p.p.m. of S0 EXAMPLE 2 (Given for comparison purpose) 10,000 Nm /h. of the outflow from a Claus unit with the following composition by volume:

H S=0.75% H O=3O% SO =0.37% H =l% COS=0 .19% COg=2.50% CS =0.05% N =65.14%

I EXAMPLE 3 (Given for comparison purpose) 10,000 Nm /h. of the outflow from a Claus unit with the following composition by volume:

H S=0.75 H 0: 30% SO =0.37% H =1% COS=0.19'% CO =2.5 0% CS =0.05% N =65.14%

are passed, at 300 C., under a pressure of 1.25 absolute atmospheres, over 10 m. of an alumina having a Na O COIZItCIIt lower than 0.1% and a specific surface of 277 m. /g.

The content of sulphur compounds of the so-treated outflow, as determined by analysis, is the following:

SO =0.35% COS=0.01%

This outflow is then contacted, at 130 C., under a pressure of 1.05 atmospheres, with a polyethylene-glycol having a mean molecular weight of 400 and containing 6.4 g. per kg. of potassium benzoate. The outflow then contains no more than 3,700 p.p.m. of acid gases. After incineration, the S content of the gas amounts to about 3,400 p.p.m.

EXAMPLE 4 (Given for comparison purpose) 18,000 Nm /h. of the outflow from a Claus unit with the following composition by volume:

H S=0.90% CO =12.00% SO =0.48% H O=29.00% COS=0.15% N =55.97% H 1.50% 1 are passed, at 300 C., under a pressure of 1.25 absolute atmospheres, over 10 m. of an alumina containing less than 0.1% of Na O and having a specific surface of 277 m. /g.

The content of sulphur compounds of the so-treated outflow as determined by analysis, is the following:

H S=1.09%; SO =0.44%; COS=0.01%

which corresponds to a molar ratio:

This outflow is then contacted, at 130 C., under a pressure of 1.05 absolute atmospheres, with polyethylene-glycol having a mean molecular weight of 400 and containing 10 grams per kg. of di-potassium phthalate. The outflow then contains 3,000 p.p.m. of H 8 and only 450 p.p.m. of S0 EXAMPLE 18,000 Nm /h. of an outflow from a Claus unit with the following composition by volume, as determined by analysis:

H,s-=0.90% co,-= 12.00% SO =0.48% H O=29.00% COS=0.15% Nz=55.97% H2=1.50% I are passed, at 300 C., under a pressure of 1.25 absolute atmospheres, over m5 of an alumina containing less than 0.1% of Na 0 and having a specific surface of 277 m. g.

The sulphur compounds content 'by volume of the sotreated outflow, as determined by analysis, is the following:

H S=l.0 9%; SO =0.44%; COS=0.01% which corresponds to a molar ratio H S s 2.48

This outflow is partially oxidized under the same conditions as those indicated in example 1 (pressure:l.15 absolute atmospheres, 750 m. /h. of air and the same catalyst). The following content in acid gases of the resulting outflow has been determined by analysis:

H 3: 1.02% SO =0.51%

which corresponds to a molar ratio:

{The outflow is then contacted, at C., under a pressure of 1.05 atmospheres with polyethylene-glycol having a mean molecular weight of 400 and containing 10 grams of di-potassium phthalate per kg. of solvent.

The outflow then contains no more than 2,000 p.p.m. of H 8 and 1,000 p.p.m. of S0 EXAMPLE 6 (Given for comparison purpose) 18,000 Nm /h. of an outflow from a Claus unit having the following composition by volume:

H23: 0.90% C0,: 12.00% SO2=0.48%'; H2O=29.00% c0s=0.1s% N2=55.97% H2=1.50%

are passed, at 300 C., under a pressure of 1.25 absolute atmospheres, over 10 m. of an alumina containing less than 0.1% of Na O and having a specific surface of 277 mP/g.

The analysis of the sulphur compounds contained in the eflluent has given the following results, by volume:

H S=1.09%; SO =0.44%; COS=0.01%

which corresponds to a molar ratio:

as S0 This outflow is partially oxidized under the same conditions as those indicated in example 1 but with the addition of a slightly greater amount of air.

The analysis of the acid gases contained in the efliuent gives the following results:

H S=0.92%; SO =0.61%

which corresponds to a molar ratio:

as S0;

This efliuent is then contacted, at 130 C., under a pressure of 1.05 atn1., with polyethylene-glycol having a mean molecular weight of 400 and containing 10 g. of dipotassium phthalate per kg. of solvent. The efliuent then contains 1,800 p.p.m. of H 5 and 2,400 p.p.m. of S0 which is an indication that it is not advantageous to proceed with a ratio H S/SO much dilferent from the preferred one.

EXAMPLE 7 Example 1 is repeated with a gas having the following composition:

H S=0.42% (by volume) H O=30% COS=0.15% CO 1.8% CS =0.04% N =66.28%

After passage over alumina, COS and CS cannot be detected and the gas contains 0.65% of H 8 and 0.21% of S0 (0.58% H 5 and 0.28% S0 after partial oxidation).

After passage through the liquid phase containing the polyethylene-glycol, the gas contains no more than 860 p.p.m. of acid gases, or 600 p.p.m. after incineration.

EXAMPLE 8 H S=0.075% COS=O.19% SO =0.037% CS =0.05%

This outfiow, which still contains a high proportion of; steam, is passed, at 300 C. and under a pressure of 1.25 absolute atmopsheres, over 10 m3 of an alumina containing less than 0.1% by weight of Na O and having a specific surface of 277 mF/g. The analysis of the sulphur compounds contained in the so-treated efliuent is as follows, by volume:

H S=O.355%; s0,=0.037%; COS=0.01%.

This eflluent is partially oxidized, at 400 C., under a pressure of 1.15 absolute atmospheres, by addition of 750 m. /hour of air, in the presence of alumina of a spe-- cific surface of 277 m. /g., on which 10% by weight of V have been deposited.

By analysis it appears that the produced eflluent has the following content of acid gases, by volume:

H S=0.26%; SO =0.13.

This eflluent is then passed through a reaction zone in which it is contacted, at 130 C. and under a pressure of 1.05 atmospheres, with a polyethylene-glycol having a mean molecular weight of 400, and containing 6.4 g. of potassium benzoate per kg. of solvent. The efiluent then contains no more than 600 p.p.m. of acid gases, which cor-, responds, after a conventional incineration, to about 480 ppm. of S0,.

What we claim is:

1. In a process for purifying a gas containing H 8, SO; and organic sulphur compounds, said process comprising reacting H S and S0 in a sulphur producing reaction with an organic solvent at 20-160 0., the improvement comprising the preliminary steps of (a) reacting said gas and steam in a first reaction zone in the presence of a catalyst consisting essentially of alumina at 250400 C. to convert said organic compounds substantially completely to H 5 while efi'ecting substantially no conversion of the S0 in the gas v to H S, resulting in a gas having an undesirably high molar ratio of H S/SO and (b) reacting resultant gas from step (a) with suflicient oxygen in a second reaction zone at 300-500 C., to convert a portion of the H 8 in said gas to S0; and to form a gas having a desirably lower molar ratio of H S/SO between 1.6 and 3.

2. A process acocrding to claim 1, wherein the gas contains from 0.01 to 4% of sulphur compounds and from 5 to 60% of steam, by volume.

3. A process according to claim 1, wherein the mixture of gas and steam contains, by volume, from 0.5 to 4% of sulphur compounds of which 0.01 to 0.3% are COS and 0.01 to 0.3% are CS the remainder being H S and S0 in a volumetric ratio of about 2:1 from 0.5 to 3%! of hydrogen and from 10 to 40% of steam, the molar ratio of H S/SO in the resultant gas from step (a) is higher than 2.4, and in which the oxygen amount in step (b) is from 0.1 to 5% of the total gas volume and is so selected that the molar ratio ins/so, in the outflow from the second reaction zone be from 1.8 to 2.4.

4. A process according to claim 1, wherein the molarratio of H S/SO in the resultant gas from step (a) is above 2.2 and oxygen is introduced into the second reaction zone in such an amount that the molar ratio HgS/SO: in the outflow from said zone be from 1.9 to 2.2.

5. A process according to claim 3, wherein the temperature in the sulphur-producing reaction zone is from 115 to 160 C.

6. A process according to claim 3, wherein in the sulphur-producing reaction zone, the organic solvent is selected among the alkylene-glycols, polyalkylene-glycols, the ethers, esters, ethers-esters of alkylene-glycols and of polyalkylene-glycols and the triesters of orthophosphoric acid.

7. A process according to claim 6, wherein in the sulphur-producing reaction zone, there is introduced in the solvent, a catalyst selected among the salts formed between metals from Groups LA and II-A and organic carboxylic acids.

8. A process according to claim 3, wherein the treated gas is an elfiuent gas from a Claus unit.

9. A process acocrding to claim 3, wherein the treated gas is an efiiuent gas from a Claus unit after treatment by passage through a liquid phase containing an inert organic solvent.

10. A process according to Claim 3 wherein the gas to be purified is Claus-unit efiluent containing on a volumetric basis, 0.l-3% H S and SO; in a ratio of about 221 respectively, 0.01-0.3% COS and 0.01-3% CS 11. A process according to Claim 1 wherein the reaction in step (b) is conducted in the presence of an oxidation catalyst.

12. A process as defined by Claim 11 wherein said oxidation catalyst comprises alumina and an oxide, in an amount of l20% by weight of the catalyst, of a metal selected from Group VA, VIA and VIII of the periodic classification.

13. A process as defined by Claim 12 wherein said metal is chromium, vanadium or iron.

14. A process as defined by Claim 1 wherein step (a) is conducted at 300-350 C.

15. A process as defined by Claim 14 wherein step (b) is conducted at 350-400 C.

16. A process as defined by Claim 14 wherein the amount of oxygen introduced into step (b) reacts sufficiently to result in a maximum oxygen content of 700 parts per million in the eifiuent from step (b).

17. A process as defined by Claim 1 wherein said alumina catalyst has an alkali metal content lower than 0.1%, a specific surface of 40-500 square meters per gram and a pore volume of from 10-80 cc./ g.

18. A process as defined by Claim 17 wherein said alumina catalyst has a specific surface of 180-350 square meters per gram.

19. A process as defined by Claim 16 wherein said. alumina catalyst has an alkali metal content lower than 0.1%, a specific surface of 40-500 square meters per gram and a pore volume of from 10-80 cc./100 g.

20. A process as defined by Claim 19 wherein said alumina catalyst has a specific surface of 180-350 square meters per gram.

References Cited UNITED STATES PATENTS 3,752,877 8/1973 Beavon 423573 X 3,794,710 2/1974 Merrill 423573 X 3,441,379 4/ 1969 Renault 423575 3,598,529 '8/ 1971 Deschamps et al. 423575 3,317,281 5/ 1967 Grerel 423576 1,895,724 1/ 1933 Miller et al 423244 X 1,710,141 1/ 1933 Benner et al 423244 FOREIGN PATENTS 340,016 12/1930 Great Britain 423244 223,904 5/1957 Australia 423244 1,329,882 5/ 1963 France.

776,319 6/ 1957 Great Britain. 1,138,437 1/1969 Great Britain.

OTHER REFERENCES Beavon et aL, Prevention of Air Pollution by Sulfur Plants," Am. Inst. Chem. Eng. Apr. 2, 1971, p. 17.

GEORGE O. PETERS, Primary Examiner U.S. Cl. X.R. 423-576 

1. IN A PROCESS FOR PURIFYING A GAS CONTAINING H2S, SO2 AND ORGANIC SULPHUR COMPOUNDS, SAID PROCESS COMPRISING REACTING H2S AND SO2 IN A SULPHUR PRODUCING REACTION WITH AN ORGANIC SOLVENT AT 20-160*C., THE IMPROVEMENT COMPRISING THE PRELIMINARY STEPS OF (A) REACTING SAID GAS AND STEAM IN A FIRST REACTION ZONE IN THE PRESENCE OF A CATALYST CONSISTING ESSENTIALLY OF ALUMINA AT 250-400*C. TO CONVERT SAID ORGANIC COMPOUNDS SUBSTANTIALLY COMPLETELY TO H2S WHILE EFFECTING SUBSTANTIALLY NO CONVERSION OF THE SO2 IN THE GAS TO H2S, RESULTING IN A GAS HAVING AN UNDESIRABLY HIGH MOLAR RATIO OF H2S/SO2, AND (B) REACTING RESULTANT GAS FROM STEP (A) WITH SUFFICIENT OXYGEN IN A SECOND REACTION ZONE AT 300-500*C., TO CONVERT A PORTION OF THE H2S IN SAID GAS TO SO2 AND TO FORM A GAS HAVING A DESIRABLY LOWER MOLAR RATIO OF H2S/SO2 BETWEEN 1.6 AND
 3. 